How do I to find average prices for used 5th wheels?

I'm going to try and give you the best advice I can give you. Search around on used unit sites, look at the guides and find an owners site for the mfg and ask what it is worth on there - THEN decide what YOU would like to pay. If you think it is a good deal, both you and the buyer feel satisfied, you got a good price. Who cares what anyone else says?